After making a surprise announcement that she's expecting her second child during her show at the Staples Center in Los Angeles Wednesday (Aug. 19), Clarkson mentioned it was Lovato's birthday the following day. Referring to Lovato as one of her girls, she gave a little history of their pop-diva friendship.

"She's always been so supportive and so cool," Clarkson said. “To see a kid that used to go to my shows have this huge, lustrous career...she’s just killin’ it. I thought I would, for her birthday, sing 'Cool For The Summer.'"